I agree about that discussion on phoenixrising being the perfect one. I've read it and it seems that one must also be taking folate with the b12 for the potassium to drop.
Missfancypants, my symptoms of low potassium were urinary frequency, excessive thirst, muscle spasms, joint pain from the spasms, twitching/jerky movements, toes curling up. If you aren't having any of those symptoms, I doubt it's low potassium. Besides 99mg is a VERY small amount of potassium. For refference, a banana has 400mg.
By the way, I also take methyl b12 and had been curious about the same thing...